The presently available Gamma Cameras are capable of acquiring images of the functioning of organs of the body. For example, parameters and images of heart beats are obtainable using gating methods. In general in gated methods, each heart beat is divided into a number of intervals of short time duration during which the heart imaging data is acquired. The amount of radiation collected during such an interval is much too low to provide a useable image. Therefore the acquisition is extended over a long period, for example 200-500 heart beats, and the gamma photons collected during corresponding intervals from each heart beat are added together. To be of any value only data from similar beats should be added. That is, beats in which the heart supposedly follows the same motion. Therefore a rejection mechanism is used to limit the data to beats that are sufficiently similar.
Gating is usually done based on the electro-cardiac signals, which are acquired by an electrocardiograph (ECG) amplifier. An ECG recorder enables visual inspection of this signal. The electrocardiogram (the output of the ECG recorder) is a graphical representation of the electrical signals obtained from the heart durig the acquisition of the signal. The signal's amplitude varies with time, graphically creating "shapes". As is well known the heart beat provides certain characteristics defined by amplitudes and differently shaped portions. One of the shaped portions is known in general as the R-wave and is part of the shape system known as "QRS Complex".
Gating is defined as synchronizing the images with a physiological signal. Multi-gating is defined as creating a set of images each synchronized with a different point during the cycle of the physiological signals. Different sub-methods use points defined, among other ways, on the basis of: the time from the last R-wave, the "phase" (fraction of R-R time, see below), or the time left to the next R-wave.
Rejection is discarding of data because the parameter does not fall within a proscribed window, as explained below. Rejection is one possible disposition of the data, with addition to other data is another disposition of the data. The decision as to which action to take is made by the decision systems.
Similar gated acquisition methods are used with other imaging modalities, most notably the nuclear magnetic resonance imaging (MRI or NMR). The invention is, however, not limited to the two mentioned modalities. In this description gamma cameras are used as an illustrative example only, whereas the inventive idea specifically applies to all imaging modalities using gated acquisition means and methods.
In the presently available systems the rejection mechanism is generally a temporal widow, wherein each heart beat is timed, usually by detecting the part of the electro-cardiac signal called R-wave which appears in every beat. If the beat length; i.e., R-wave to R-wave interval (R-R time for short) is outside the window then at least one beat is rejected. If a beat is too short both it and the following beat are usually rejected. This assumes both beats are not "normal". One ends abnormally and the next starts abnormally.
This rejection mechanism allows the use of the window for acquiring a series of images representing a "normal" beat, as defined by the prior art systems. The normal beat is defined by the prior art systems as a beat with an R-R time in the window where the window is usually set around the most common R-R time found during a test or learning period. Thus, the definition of "normal" is subjective as each patient has a heart beat that is normal for that person at the time of the test or learning period.
There are several drawbacks or shortcomings in the prior art systems. These drawbacks are generally caused by a one-dimensional analysis of the heart beat. Among the drawbacks in prior art systems are:
All beats within the window are accepted (not rejected) as they are considered "normal". All beats that pass through the window are not necessarily "normal" and the images are degraded by inclusion of "aberrant" beats ("normal" in R-R time but not otherwise).
There is no discrimination between different types of short (or long) beats. For example, the physiology of a certain type of short beat (premature ventricular beat or PVB) frequently causes the next beat to be longer than "normal" to compensate for the shortness. These compensatory beats should be, but are not rejected. Other types of short beats (for example premature atrial beats, or PAB) are not followed by such compensatory beats. The rejection mechanism is not optimal if it does not discriminate between the different types of short beats. The same argument holds for "long" beats.
A lot of information is lost (and consequently a lot of radiation dose to the patient is not utilized) by rejecting all "abnormal" beats.
This last shortcoming is especially serious, not only because of the "unused" or "wasted" radiation dosage but also because it results in images not truly representative of the cardiac condition. The cardiac parameters, such as cardiac output are based on the images created for a "normal" cycle. When that "normal" cycle is statistically not sufficiently representative (although it is based on the most prevalent R-R time, it may still account for less than 50% of the heart beats, say), then the parameters so obtained do not yield enough information about the patient's true cardiac condition.
This and the first two shortcomings are emphasized in the rest versus exercise comparison test, which is becoming increasingly more accepted as a procedure for cardiac examination. Many patients exhibiting essentially "normal" beats at rest start exhibiting abnormalities, such as abernant beats or an increased prevalence of PVBs, during the stress of an exercise test.
Two other approaches are being tried today to overcome the drawbacks of presently available prior art systems. The first is sometimes referred to as "list mode" or "serial" acquisition (or several other names) and consists of listing all nuclear events, their location and timing, and later constructing the set of images for the cardiac cycle off-line. The rejection is done manually. This is a time consuming method and reduces the throughput of the clinic. At the same time it is still not optimal as it still uses only the R-R times as the basis for rejection.
The second approach consists of having several R-R windows. In some cases, where a single very specific cause creates all abnormal beats this sufficies. However, in most cases short heart beats may have widely differing electrical and mechanical shapes, determined usually by the position of the abnormal site (locus) of the "firing" mechanism and therefore this second approach does not usually overcome the drawbacks of the available systems.
It should be noted that additional information about the heart's condition does exist. This is the electrocardiogram (ECG) tracing, which follows the electrophysiological signals from the heart. These signals are determined by the route and speed of the electrical pulses which cause the heart muscles to contract and expand. These pulses, in travelling from the location of the "firing" mechanism to and through the heart muscles, radiate electromagnetic waves which are detected by the ECG's electrodes.
The pulse causing ventricular contraction is detected as part of the signal called the "QRS complex". It is distinctive and provides information about the ventricular contraction phase and about many defects in the ventricular contraction. Other parts of the signal teach about other phases, e.g. atrial contraction (P-wave), ventricular expansion (T-wave) etc. However, this other information is not usually used for gating purposes in current systems.
Therefore, it is an object of this invention to provide a system that uses ECG-based information in addition to the R-R time (e.g. the QRS shape) to obtain more data about each heart beat and to thereby base the decision about using or rejecting the acquired imaging data on more solid ground. At the same time it enables selecting more than one beat type, where again the type of heart beat is decided on the basis of both R-R time and QRS shape. The system provided herein can be also used to decide acceptance or rejection in conjunction with imaging acquisition gating some other physiological signal, e.g. a spirometric signal, measuring breathing.